Book Image

LLVM Essentials

By : Mayur Pandey, Suyog Sarda, David Farago
Book Image

LLVM Essentials

By: Mayur Pandey, Suyog Sarda, David Farago

Overview of this book

LLVM is currently the point of interest for many firms, and has a very active open source community. It provides us with a compiler infrastructure that can be used to write a compiler for a language. It provides us with a set of reusable libraries that can be used to optimize code, and a target-independent code generator to generate code for different backends. It also provides us with a lot of other utility tools that can be easily integrated into compiler projects. This book details how you can use the LLVM compiler infrastructure libraries effectively, and will enable you to design your own custom compiler with LLVM in a snap. We start with the basics, where you’ll get to know all about LLVM. We then cover how you can use LLVM library calls to emit intermediate representation (IR) of simple and complex high-level language paradigms. Moving on, we show you how to implement optimizations at different levels, write an optimization pass, generate code that is independent of a target, and then map the code generated to a backend. The book also walks you through CLANG, IR to IR transformations, advanced IR block transformations, and target machines. By the end of this book, you’ll be able to easily utilize the LLVM libraries in your own projects.
Table of Contents (14 chapters)
LLVM Essentials
About the Authors
About the Reviewer

Register allocation

The next step of the code generator is register allocation. As we saw in the previous example, some of the registers being used were virtual registers. Register allocation is the task of assigning physical registers to these virtual registers. In LLVM, the virtual registers can be infinite in number, but the numbers of physical registers are limited depending on the target. So, by register allocation, we aim at maximizing the number of physical registers being assigned to virtual registers. We must note that the physical registers are limited in number, so it is not always possible that all the virtual registers can be assigned a physical register. If there is no physical register available at some point and we need a physical register for a variable, we might move a variable that is present in physical register to main memory and thus assign the freed register to the variable we want. This process of moving a variable from physical register to memory is called spilling...